Steam generating unit



Nov. 12, 1963 E. c. MILLER 3,110,291

STEAM GENERATING UNIT Filed Jan. 16, 1962 2 Sheets-Sheet 1 2p g g a g 78INVENTOR. z EasrZe C. UZI'ZZe'r H oney Nov. 12, 1963 E. c. MILLER 3,

'STEAM GENERATING UNIT Filed Jan. 16, 1962 2 Sheets-Sheet 2 ,2 h. Il

I I V 4 INVENTOR.

'jarZe C. miller United States Patent 3,110,291 STEAM GENERATING UNITEarle C. Miller, Worcester, Mass, assiguor to Riley Stoker Corporation,Worcester, Mass, a corporation of Massachusetts Filed Jan. 16, 1962,Ser. No. 166,573 Claims. (Cl. 122-479) This invention relates to a steamgenerating unit and more particularly to apparatus arranged to producesteam by the burning of a combustible fuel.

In the generation of steam, particularly for the purposes of powergeneration in a steam turbine, the control of the temperature of thesuperheated steam is a considerable problem. This comes about because ofthe need to maintain this temperature as close to a predetermined designtemperature for efficient operation without exceeding it and thusencountering the problem of failure of materials. Another problem whichis often encountered with large steam generating units is that, duringthe combustion of the fuel, nitrous oxides are formed which areprojected into the air from the stack and are thought to produce aconsiderable amount of the irritants in the air in certain parts of theworld. These and other difficulties experienced with the prior artdevices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the present invention to'provide a steam generating unit having a novel means for maintainingsuperheat at a constant, pre-determined value.

Another object of this invention is the provision of a steam generatingunit having a novel means for reducing the amount of nitrous oxide inthe products of combustion.

A further object of the present invention is the provision of a steamgenerating unit having means for controlling superheat and at the sametime regulating the amounts of nitrous oxide in the products ofcombustion.

With these and other objects in view, as will be apparent to thoseskilled in the art, the invention resides in the combination of partsset forth in the specification and covered by the claims appendedhereto.

The character of the invention, however, may be best understood byreference to one of its structural forms, as illustrated by theaccompanying drawings in which:

FIG. 1 is a vertical sectional view of the steam generating unitembodying the principles of the present invention,

FIG. 2 is an enlarged elevational view of a portion of the apparatustaken on the line 11-11 of FIG. 1,

FIG. 3 is a schematic view of the apparatus showing the mode ofoperation at high load, and

FIG. 4 is a schematic view of the apparatus showing the mode ofoperation at low load.

Referring first to FIG. 1, wherein are shown the general features of theinvention, the steam generating unit, indicated generally by thereference numeral 19, is shown as consisting of a furnace 11 and aboiler 12 suspended from a supporting structure 13. The furnace is ofthe elongated vertical type and is provided with a front wall 14, a rearwall 15, side walls 16, a roof 17, and a bottom 18 defining a combustionchamber 19. An inwardlydirected abutment 21 extends inwardly from thefront wall 14, while a similar abutment 22 extends inwardly from therear wall 15, the abutments serving to divide the combustion chamber 19into a lower high-temperature cell 23 and an upper portion 24. Mountedin a downwardly-directed surface of the abutment 21 is adirectional-flame burner 25, While a similar burner 26 is mounted on adownwardly-directed surface of the abutment 22. The burners and generalfurnace arrangement are those shown in the patent of Miller No.2,947,289.

At the upper part of the furnace, the rear wall 15 is provided with aforwardly-directed nose 27. Between the upper surface of the nose 27 andthe roof 17 is defined a horizontal pass 28 leading to a gas off-takeopening 29. The off-take opening leads to a back-pass 31 having avertical dividing wall 32 separating the back pass into a forwardportion 33 and a rearward portion 34. The lower end of the back pass 31is connected through a regenerative air heater 35 to a breaching 36leading to the stack.

The boiler 12 is provided with a steam-and-water drum 37 containing theusual steam purifying equipment and connected by downcomers 38 toheaders 39 and 41 underlying the bottom 18 of the furnace. Water-walltubes extend upwardly from the header 39 and along the surface of theback wall 15 along the surface of the abutment 22 and the nose 27 toterminate at the steam-and-water drum for the release of steam thereto.In a similar manner, water-wall tubes extend upwardly from the header 41along the front wall 14, overlying the abutment 21 and also providingprotection for the roof 17. The downcomers 38 are also connected toheaders 42 underlying the side wall 16 and connected to water-wall tubeswhich extend over the surface of the side walls and eventually terminatein a steam-and-water drum 37. An economizer 43 extends entirely acrossthe back pass 31 lying in both the bottom parts of the forward portion33 and the rearward portion 34. Steam tubes 44 leave the upper part ofthe steam-and-Water drum 37 and some of them support the dividing wall32, while others line the back wall of the rearward portion 34 of theback pass. In either case, they are connected to the lower end of a lowtemperature convection superheater 45 which lies in the rearward pass34.

The upper part of this superheater is connected by a pipe 46 to thelower end of a radiant intermediate temperature superheater 47 which ismounted adjacent the front wall 14 of the furnace between the abutment21 and the roof 17. The upper end of this last-named superheater isconnected by a pipe 48 to the forward part of a high-temperaturesuperheater 49. The superheater 49 is in the form of pendant platenswhich terminate in a short convection section lying in the horizontalpass 28 and which is ultimately connected to a superheated steam header51.

A low-temperature convection reheater 52 lies in the forward portion 33of the back pass and is connected at its upper end to a high-temperatureconvection reheater 53 which lies in the horizontal pass 28 adjacent thegas offtake opening 29. This high-temperature reheater discharges into areheated steam header 54.

A forced draft fan 55 provides air to the air heater 35 which, in turn,discharges preheated air through a venturi duct 56, which duct isprovided with a damper 57 capable of dividing the air between a plenumchamber 58 for supplying air to the burners 2S and 26 and an overfireair duct 59. A duct 61 leads from the plenum chamber 58 to the burners26 arranged on the undersurface of the abutment 22 and this duct isprovided with a damper 62.

. In a similar manner, a duct 63 leads from the plenum chamber 58 to theburners 25 on the undersurface of the abutments 21, this duct beingprovided with a damper 64.

The damper 64 is connected by a linkage to a hydraulic 1 linear actuator65 which is connected by lines 66 and 67 to the output of a main control68. The input of the main control 63 is connected by a line 69 to atemperature measuring device 71 located in the reheated steam header 54.A similar temperature measuring device 72 is located in the superheatedsteam header 51 and is connected by a line 73 to the input side of themain control 68. The damper 62 in the duct 61 is also connected througha linkage to a hydraulic linear actuator 74 which is con trolled by themain control 68 through lines 75 and 76. The overfire air duct 59 isconnected by a first branch duct 77 to an overfire air opening 7 8located in the front wall 14-.of the furnace just above the abutment 21.The duct Patented Nov. 12, 1963.

59 is also connected through a branch duct 75 to a similar overfire airopening 81 located on the rear wall 15 just above the abutment 22. Inthe duct 77 is located a damper 82 which is connected through a linkageto a hydraulic actuator 83 which is controlled by the main control 63through the medium of lines 84 and 35. In a similar manner, the duct 79is provided with a damper 86 whose setting is controlled by a hydrauliclinear actuator 37 which is connected to the main control 63 by lines 88and 89.

At the bottom of the rearward portion 34 of the backpass 31 is located aset of dampers 91 connected by linkages to a hydraulic linear actuator92 which is connected for control to the main control '68 by means oflines 93 and 94. At the bottom of the forward portion 33 of the backpassare located dampers 95 which are connected by linkages to a hydrauliclinear actuator 96 which is connected to the main control by means oflines 97 and 98. The damper 57, which controls the division of theincoming air between the plenum chamber 58 and the over fire air duct59, is connected -by linkages to a hydraulic linear actuator 99 which isconnected by lines 101 and 162 to the output of the main control 63.Pressure plugs 103 and 104 are mounted in the venturi duct 56 and areconnected by lines 1695 and 196 to the input side of the niain controlas, the location in this manner giving a pressure differential betweenthe two lines 165 and 166 indicative of the rate of flow of air into thefurnace and, therefore, of the load.

In the usual manner, a desuperheater 1425 is connected in the pipe 46between the low temperature superheater 45 and the radiant intermediatetemperature superheater 47. The amount of water injected into the steamis controlled from the main control 68 by a line 1636 extending betweenthe main control and the desuperheater. It will be understood that themain control '68 is of the usual type which receives signals fromvarious sources such as devices measuring load, such as the plugs 103and 194, and devices for measuring steam temperatures, such as devices 71 and 72, and converting them into signals which pass through thevarious lines to actuators which operate to correct tendencies forchange in the temperatures of superheat and reheat. The nature of thiscontrol is not part of the present invention and its design andconstruction is well known in the art.

Referring to FIG. 2, it can be seen that the opening 78 through thefront wall 14 of the furnace is formed by bendin alternate water-walltubes rearwardly of adjacent water-wall tubes to provide inter-tubeopenings or slots through the wall. The opening 81 on the rear wall 15is similarly constructed.

l The operation of the apparatus will now be readily understood in viewof the above description and particularly in view of FIG. 3 and FIG. 4.The burners 25 and 26 operate in the usual manner to produce a mixtureof fuel and air which burns to form a mass of products of, combustion1G7. It will be understood that most of the visible flame portion ofthis gas mass will terminate slightlyabove the abutments 21 and 22because of the nature of the particular furnace involved and thepresence of" the high-temperature cell 23 in which a considerable partof the combustion can be completed. The products of combustion flowupwardly through the combustion chamber -19.around the nose 27, throughthe horizontal pass 28, and through the gas iofiitake 29 into thebackpass .3 1. It is divided between the forward portion 33 and therearward portion 34, depending on the settings of the dampers 91 and 55.Under certain conditions, some of the gas may be directed through aby-pass section (not shown) of the back passin which there are nosuperheater or, reheater elements. The gas leaves the backpass, flowsthrough the air heater 35 where it serves to heat the incoming air, andthen passes into the breaching 36. Flowing in this manner, water isheated and steam is generated, superheated, and reheatedin the usualmanner by the gas mass transmitting heat by radiation to the water-Walls and to the radiant superheater 47, transmitting heat to thesuperheater 49 by a combination of radiation and convection, andtransmitting heat mainly by convection to the reheater sections 52 and53 as well as the superheater i S. Heat is recovered in the economizer43 as well as in the air heater 35 to give the unit optimum effi ciency.If the steam generating unit were operated only in this way at variousloads, the superheat passing into the superheated steam header 51 on itsway to the turbine wouldrdr-oop, i.e., the curve of superheat versusload wouldbe inclined with lower superheat temperatures at lower loads.The amount of air passing into the burners 25 and 26 is controlled bythe dampers 64 and 62, re-

spect-ively, so as to cause the gas mass 107 to move up the back wall 15of the furnace or up the front wall 14. This is done through the mediumof the actuators 65 and 74 under the regulation of the main control 68through the medium of lines '66, 67, 75 and 76. This type of flameplacement is shown and described in the aforementioned patent of MillerNo. 2,947,289. Generally speakiug, this effect is produced by causing agreater flow;

'trol 68 by means of the pressure plugs 103 and 10 4 and their lines Hi5and 1%, the dampers are set to cause a considerably greater flow of airfrom the duct 77 through the opening 78 in the front wall than flowsthrough the duct 79 and the opening 81 in the rear wall. The effect ofthis is to force the gas mass 107 toward the rear wall.

of the furnace. At high load, the total amount of air required by all ofthe burners 25 and 26 is quite high, and this is regulated either by thedampers 62 and 64 operating together, or by control of the motorassociated with the forced draft fan 55 going at a faster rate. At thesame time, the total amount of overfire air passing through the openings78 and 81 is also higher at high load. However, -the division will besuch that the overfire air coming from the front wall through theopening 7 B will domi-' nate the overfire air coming through the opening81 in the rear wall and the gas mass will be pushed toward the rearwall. This overfire air, of course, has the elfect of protecting thefront and rear walls and of producing a complete burning of nitrousoxide in a manner which will be described below. i 7

At low load,the dampers 62 and 64 will be set to force the gas mass upthe front wall of the furnace, for instance, by causing the air flowingthrough the burner 25 to be much greater than that flowing from theburner 26 so that the flame is thrown toward the front wall. It will beunderstood that this same type of flame placement can also beaccomplished by causing the vanes in the inter-tube burners to occupysuitable positionsrelative to one an-.

other. At the same time, the dampers 86 and 82 are set to cause the airproceeding fromthe opening 81 in the) rear wall to dominate thesituation, thus forcing the gas flow also toward the front wall. Thismovement of the flame from the front to the rear of the furnace asselected by the main control 63 has the effect of maintaining thesuperheat in the superheated. steam header 51 at a con- This isbeoauseathigh load,

27; the eifect is a longer residence time in the furnace and coolergases passing over the convection surfacesof the boiler. At the sametime, the flame is furthest away from the radiant superheater 47 andheat transfer to the radiant superheater is at its lowest value. On theother hand, at low load, as is evident, in FIG. 4, the flame passes upthe front wall 14 where it transmits the greatest amount of heat byradiation to the superheater 47 and takes the shortest path around thenose 2.7; that is to say, has the shortest residence time in the maincombustion chamber 19, thus producing highest temperature of gasespassing into the convection passes. it will be understood that atvarious loads between high load and low load the flame may occupyintermediate positions between the front and rear walls of the furnace.This method of controlling the superheat might, in a practical boiler,be combined with other methods. For instance, if, at any time, at higherloads the temperature of superheat exceeds the predetermined valuedespite the best efforts by the flame placement technique, thetemperature may be reduced by a signal passing through the line 166 tothe desuperheater 3195. Additionally, the relative temperatures of thereheated steam in the reheated steam header 54 and the superheated steamin the superheated steam header 51 may be adjusted by regulating theproportions of gas which pass through the forward portion 33 of the backpass over the convection reheater 52 and the rearward portion 34 of theback pass over the convection superheater 45. Additional lowering of thesuperheat may be accomplished in a well-known manner by shunting some ofthe gases through the dead by-pass section of the back pass so that theypass over neither the reheater 52 or the superheater 45.

Now, the damper 57 is regulated by the actuator 99 operating from themain control 68 through the lines 1G1 and 102. This damper divides theincoming air between -the burners 25 and 26 on the one hand and theoverfire air openings 78 and 81 on the other hand. At high load,although considerable air is needed for combustion through the burners,the damper is set to permit the burners to have a lower proportion ofthe air. On the other hand, at low load, where air is necessary in theburners to maintain ignition, the greater proportion of air goes to theburners. At the same time, it is not necessary to provide as greatamount of overflre air through the openings 78 and 31.

In general, then, the present invention involves dividing the combustionair between the overflre air openings which are above the abutments 21and 22 and the directional-flame burners mounted on the undersurfaces ofthe abutments. With ordinary operation of the furnace, the percentage ofnitrous oxide in the flue gases increases with load in a more or lessstraight line relationship. It has been discovered that the slope ofthis curve (or the absolute value of the nitrous oxide) can he reducedby lowering the velocity of the fuel and air to the burner. At highload, where the velocity of gases through the burner would normally bequite high, some of the combustion air is introduced independently intothe furnace as overfire air, this point of introduction being quiteseparated from the portion in which air is introduced which combinesimmediately with the fuel from the main fuel gun. At low load, where itis necessary to mix the air immediately with the fuel to maintainignition, a considerable portion of the air would be introduced throughthe main burners in the usual way rather than as overfire air in aseparate stream. At low load, the unit will be operating on the lowerpart of the nitrous oxide curve where the formation of nitrous oxide isquite low. In the preferred embodiment, at low load the greater amountof the air will go through the burners and the flame placement will besuch that the flame will go up the front wall to provide longerresidence time and be closer to the radiant superheater. by the greaterproportion-of the overflre air being introduced to the back wall toassist in pressing the flame forwardly. At high load, however, a greateramount of air is introduced as overfire air and the burners are set toproduce a flame which would be positioned toward the rear wall producinglonger residence time and less radiant heat transmission to the radiantsuperheater. The

This will be assisted overfire air is biased so that the front wallwould receive the greatest stream of overfire air to assist inpositioning the flame to the rear of the furnace.

It can be seen that at high load, when there is a much greater mass ofgases which will be more diflicult to position, a greater amount ofovenflre air is used, thus assisting in pushing the flame to the rear ofthe furnace. At low load, where the mass of gases is smaller and it iseasier to position the flame, the overfire air is smaller in amount,also, and is not needed in the flame-positioning to as great an extent.At the same time, at high load where nitrous oxide is a problem, the airvelocity through the burner is reduced, thus reducing the nitrous oxideformation without loss of completeness of combustion because of theoverfire air being provided at that load.

It is obvious that minor changes may be made in the form andconstruction of the invention without departing from the material spiritthereof. It is not, however, desired to confine the invention to theexact form herein shown and described, but it is desired to include allsuch as properly come within the scope claimed.

The invention having been thus described, what is claimed as new anddesired to secure by Letters Patent, is:

l. A steam generating unit, comprising (a) front, rear, and side wallsdefining a verticallyelongated furnace,

(b) a pair of abutments on opposed walls defining a high-temperaturecell at the lower end of the furnace,

(c) a nose on one of the opposed walls defining a gas off-take at theupper end of the furnace,

(d) a fuel-burning apparatus mounted in the high-temperature celladapted to form an upwardly-directed flame,

(e) overfire air openings located on both opposed walls above theabutments,

(f) a source of air including proportioning dampers connected to theapparatus and to the openings,

(g) -a radiant superheater associated with the other of the opposedwalls,

(It) and a control means responsive to an increase in the temperature ofsuperheated steam to decrease the proportion of air flowing to theopenings on the said one of the opposed walls relative to that flowingto the openings on the said other of the opposed walls to cause the saidflame to change to a position between the said opposed walls such thatthe temperature of superheated steam will be maintained at apredetermined value.

2. A steam generating unit as recited in claim 1 wherein thefuel-burning apparatus consists of vane-type directional-flame burnersmounted on the undersurfaces of the abutments.

3. A steam generating unit as recited in claim 1 wherein a back passcontaining a convection superheater is connected to the said gasoff-take.

4. A steam generating unit as recited in claim 1 wherein means isprovided to regulate the division of the air bet-ween the overfire airopenings and the fuel-burning apparatus.

5. A steam generating unit, comprising (a) front, rear and side wallsdefining a vertically-elongated furnace,

(b) a convection pass connected to the upper end of the furnace,

(c) a radiant superheater associated with one of two opposed walls,

(a') a fuel-burning apparatus mounted in the lower end of the furnaceand adapted to form an upwardlydirected flame,

(e) overfire air openings located in the said opposed walls above thefuel-burning apparatus,

(f) a source of air including proportioning dampers connected to theapparatus and to the openings,

(g) a first means for regulating the apparatus so that the flame isdirected along the other of the opposed walls at high load and along thesaid one of the 0pposed walls at low load,

(h) and a second damper means dividing the total flow of air to theopenings so that a higher proportion flows to the openings on the saidone of the 5 opposed walls at high load to assist the first means indirecting the flame along the other of the said opposed walls and sothat a, higher proportion flows to the openings ont the othertof theopposed Walls at low load to assist the first means in directing theflame along the said one of the opposed walls.

References Cited in the file of this patent UNITED STATES PATENTS2,869,519 Schroedter et al J an. 20, 1959 2,947,289 Miller Aug. 2, 1960FOREIGN PATENTS 724,934 Great Britain Feb. 23, 1955

1. A STEAM GENERATING UNIT, COMPRISING (A) FRONT, REAR, AND SIDE WALLSDEFINING A VERTICALLYELONGATED FURNACE, (B) A PAIR OF ABUTMENTS ONOPPOSED WALLS DEFINING A HIGH-TEMPERATURE CELL AT THE LOWER END OF THEFURNACE, (C) A NOSE ON ONE OF THE OPPOSED WALLS DEFINING A GAS OFF-TAKEAT THE UPPER END OF THE FURNACE, (D) A FUEL-BURNING APPARATUS MOUNTED INTHE HIGH-TEMPERATURE CELL ADAPTED TO FORM AN UPWARDLY-DIRECTED FLAME,(E) OVERFIRE AIR OPENINGS LOCATED ON BOTH OPPOSED WALLS ABOVE THEABUTMENTS, (F) A SOURCE OF AIR INCLUDING PROPORTIONING DAMPERS CONNECTEDTO THE APPARATUS AND TO THE OPENINGS, (G) A RADIANT SUPERHEATERASSOCIATED WITH THE OTHER OF THE OPPOSED WALLS,